The ion funnel has become a well established means for efficient transmission of ions in various ion sources including atmospheric pressure ionization (API) ion sources. Ions are focused by the effective potential applied to a stack of RF (e.g., ring) electrodes with gradually reducing apertures. The smallest (i.d.) aperture at the ion funnel exit, termed the conductance limit aperture, defines the gas flow exiting an ion funnel to the next stage of differential pumping. Limitations of current ion funnel configurations are well known in the art. For example, transmission of low mass ions, e.g., m/z<˜100, is typically inefficient. High m/z, large molecular, ion transmission can also be inefficient due to less effective ion focusing and reduced acceptance by the next stage following the ion funnel. Exit apertures cannot be reduced below about 2 mm currently as increased penetration of the on-axis RF field (so-called “parasite RF field”) along the ion funnel axis near the exit of the ion funnel deteriorates ion funnel performance, causing reduced ion transmission efficiency of both low and high m/z ions. In addition, high RF field intensities increase dissociation and fragmentation of analytes of interest as a consequence of fast ion motion and ion collisions with molecules of surrounding carrier gas. Further, operating pressure increases above about 5 Torr are also problematic since the efficiency of RF focusing is reduced, while factors leading to ion losses, including space charge expansion and drag forces from gas flows, are increased. While a DC-only conductance limit electrode installed immediately after the last RF focusing ring can slightly reduce the exit aperture i.d. and RF fields downstream from the funnel, the i.d. of the DC-only aperture still cannot be reduced without reducing transmission of desired ions.
Accordingly, alternatives to standard ion funnel designs are needed that provide suppression of parasite RF fields, increased transmission efficiencies for low and high m/z ions, reduced RF activation of ions, and smaller apertures for the conductance limit electrode thereby providing decreased gas loads to subsequent pumping stages.